This invention relates to an arm particularly suited for the operation of relays. The arm is controlled by electricity and has a first fixed end and a second end which is movable mainly perpendicular to the length direction of the arm. The arm comprises two mainly parallel elongated portions separated by an air gap or some other heat insulating material, so that at least one of the portions may be heated by electricity in order to effect said movement by longitudinal heat expansion with respect to the other portion.